1. Field of the Invention
The present invention relates to a digital servo-control apparatus for digitally controlling an AC servomotor and, more particularly, relates to apparatus adapted for preventing the output torque of the servomotor from fluctuating.
2. Description of the prior art
In order to avoid difficulties involved in analog control systems, a digital servo-control apparatus has recently come into use. In the digital servo-control apparatus, the target value and the feedback value are provided as digital values and the difference therebetween is calculated by a digital computer. Further, the command value according to the difference is provided as a digital value and the controlled amount (rotational angle, rotational velocity, or load current) to a controlled system, i.e., a servomotor is digitally controlled by the command value. Such servo-control apparatus, in general, have the feedback loops for a position, a velocity, and a current.
In the current feedback loop of such feedback loops, the load current is detected by a current transformer (CT), the output thereof is amplified by an analog amplifier, and the output thereof is sampled at intervals of a predetermined period to be digitized. The detected current of each phase is subjected to a dq-transformation. The load current supplied to the servomotor is feedback-controlled so that the resultant d-axis component and q-axis component of the detected current become equal to the respective target values.
The d-axis component of the load current represents the reactive current, while the q-axis component is proportional to the output torque of the servomotor when the servomotor is a synchronous motor and the strength of the exciting magnetic field is constant. Accordingly, when the servomotor is a synchronous motor, the feedback control of the load current is executed so that the d-axis component of the detected load current becomes zero and the detected q-axis component becomes equal to the target current proportional to the target output torque.
As described above, the d-axis component and the q-axis component have only DC components, respectively, regardless of the electrical angle, i.e., the phase angle of the load current. Accordingly, the dq-transformation method has an advantage that the current control to the servomotor becomes easier. Here, the electrical angle is defined as the angle formed between the exciting magnetic field and the reference axis of the armature coil in the case of a synchronous motor. More concretely, the electrical angle is, for example, the rotational angle of the revolving magnetic field in a revolving-field type synchronous motor, the rotational angle of the armature in a revolving-armature type synchronous motor, or the rotational angle of the revolving magnetic field referenced from the primary side (stationary coordinate system) in an induction motor.
In performing the above described dq-transformation of the detected load current and the inverse dq-transformation for getting a command value of current from the command values of the dq-components, the electrical angle must be given. The electrical angle is generally assumed to be constant during the control period of the velocity feedback loop.
Therefore, in the conventional apparatus, the same assumed electrical angle is used in both dq-transformation of the detected load current and inverse dq-transformation for getting the command value. However, the actual electrical angle at which the command value is output is advanced with respect to the assumed electrical angle because of computing time by the digital computer. Accordingly, the conventional apparatus has a problem that the difference between the actual electrical angle and the assumed electrical angle used in the control of the servomotor becomes large and, hence, it becomes unable to obtain an accurate command value of the phase current to be controlled. Because of this, the conventional system has a problem that the output torque of the servomotor tends to fluctuate.
The d-axis component and the q-axis component of the load current are essentially direct currents independent of the frequency of rotation of the revolving magnetic field or the revolving armature.
However, it has been found by the present inventors that the d-axis component is, in reality, not zero but has a DC component, a fundamental wave with the same frequency as the frequency of rotation of the revolving magnetic field or the revolving armature, and a second harmonic, as shown in FIG. 7, and the q-axis component has a DC component not corresponding to the output torque of the servomotor, a fundamental wave with the same frequency as the frequency of rotation of the revolving field or the revolving armature, and a second harmonic, as shown in FIG. 8.
In the feedback control, the load current is controlled according to the instantaneous values of the d-axis component and the q-axis component of the load current detected at intervals of a predetermined period and the target values of the same. Therefore, the power-factor decreases when there is an error in the DC component of the d-axis component of the detected load current, and the power-factor varies when there is a fluctuation in the d-axis component. Further, there is such a problem that the response speed tends to vary when there is an error in the DC component of the q-axis component and the output torque tends to fluctuate when there is fluctuation in the q-axis component.
Further, the sampling of the load current in the above described current feedback loop is performed only once when the PWM voltage applied to the servomotor is zero as shown in the timing chart of FIG. 11, or at the middle point of the control period of the current feedback loop. Even if the sampled value of current happened to be abnormal, the sampled value is used as the feedback value of the current feedback loop and calculation of the command value of current is performed according to the sampled value. Therefore, the conventional servo-control has a problem that the PWM voltage swings and the torque or velocity variation becomes greater.